Type Conversion

See equivalent in the Reactive stack

Some annotated controller method arguments that represent String-based request input (such as @RequestParam, @RequestHeader, @PathVariable, @MatrixVariable, and @CookieValue) can require type conversion if the argument is declared as something other than String.

For such cases, type conversion is automatically applied based on the configured converters. By default, simple types (int, long, Date, and others) are supported. You can customize type conversion through a WebDataBinder (see DataBinder) or by registering Formatters with the FormattingConversionService. See Spring Field Formatting.

A practical issue in type conversion is the treatment of an empty String source value. Such a value is treated as missing if it becomes null as a result of type conversion. This can be the case for Long, UUID, and other target types. If you want to allow null to be injected, either use the required flag on the argument annotation, or declare the argument as @Nullable.

As of 5.3, non-null arguments will be enforced even after type conversion. If your handler method intends to accept a null value as well, either declare your argument as @Nullable or mark it as required=false in the corresponding @RequestParam, etc. annotation. This is a best practice and the recommended solution for regressions encountered in a 5.3 upgrade.

Alternatively, you may specifically handle e.g. the resulting MissingPathVariableException in the case of a required @PathVariable. A null value after conversion will be treated like an empty original value, so the corresponding Missing…Exception variants will be thrown.